The invention relates to the mounting of sensors of physical quantities capable of working in harsh environments.
The mounting generally consists of the transfer of a micro-machined sensor to a base provided with electrical connection pins. The sensor is made, for example, out of several machined silicon wafers comprising mechanical elements (diaphragms, beams, seismic masses, etc), electronic elements (capacitor plates or strain gauges in particular), and metal contact pads used for electrical connection with the pins of the base when the sensor is fixed to the base.
Classically, the sensor is bonded or brazed by its rear face to the base, in a central part of this base that is surrounded by connection pins that go through the base. The connection pads of the sensor, on the front face of this sensor, are connected by bonded wires between the connection pads and the tips of the connection pins that emerge from the surface of the base.
This approach is costly, in terms of both manufacturing time and the cost of the automatic soldering machine.
The invention is aimed at proposing a less costly solution that has good qualities of mechanical resistance and greater compactness, and can be used in a large number of applications, including especially pressure sensors and accelerometers.
The invention proposes a sensor of physical quantities comprising at least one micro-machined wafer provided with conductive connection pads on a main face, and a base provided with conductive connection pins, the main face being turned towards the base and each connection pad facing a corresponding pin end, an electrolytic deposit of metal or metals coating the end of each pin and the corresponding connection pad so as to set up a rigid fastening between this end and the pad.
The term xe2x80x9celectrolytic depositxe2x80x9d is understood to mean a metal deposit (an alloy of metals or the deposition of several metals) on a conductive zone, obtained by the migration of metal ions coming from a liquid solution. The migration may be prompted either by the passage of an electrical current (in a classic electrolytic bath with current lead-in electrodes), or by chemical reaction (using what is called electroless deposition).
The method of manufacture according to the invention therefore consists in:
preparing an active sensor part and a base, the active part comprising at least one wafer provided with connection pads on a front face, and the base being provided with conductive pins whose ends are arranged spatially so that each end leans against a respective pad of the wafer when the front face of this wafer is brought closer to the base,
holding the plate against the base and dipping the plate and at least the pin ends into an electrolytic bath, and carrying out an electrolytic deposition of conductive metal on the pin ends and the pads with a metal thickness sufficient to provide for rigid fastening between the pins and the pads by the deposited metal.
The electrolytic deposition on the pads and on the pins achieves, so to speak, a solder with soldering metal between these pads and the pins, and the resistance of this solder in a harsh environment is far higher than the resistance that would be obtained if a simple conductive bonder were used between the pins and the pads. Furthermore, this joining is done without any need take the pads to high temperature as would be the case with true soldering or brazing. Furthermore, the operation of fastening by electrolytic deposition is done without mechanical strain between the pads and the pins.
This method can be implemented collectively for batches of sensors, without involving costly machines such as those used to carry out automatic  less than  less than wire-bonding greater than  greater than .
Above the surface of the base, the pins may have a substantial height, for example a height of some millimeters for a base with a diameter of one centimeter, so that the sensor is suspended in a relatively flexible way owing the flexibility proper to the pins. It is therefore less subjected to the strains that may be borne by the base, especially the strains due to heat expansion, impacts, vibration, etc.
Preferably, the connection pins and the connection pads lined with electrolytic metal are lined with an insulating layer to eliminate the risks of shorting or current leakage paths between pins when they are in a liquid or gas environment that is not perfectly insulating (moist air or saline air or water for example).
The insulation layer may be produced by electrolytic oxidization or nitrization, or by a second deposit of conductive metal and an oxidization or nitrization of this second deposit. It can also be made by the deposition of mineral insulator by chemical decomposition, possibly plasma-assisted. For less severe environments, the deposition of an insulating organic layer may be considered.